Porcine reproductive and respiratory syndrome (PRRS) is the most economically significant disease of swine worldwide. It is characterized by late term reproductive failure in sows and severe pneumonia in neonatal pigs. The PRRS virus (PRRSV) consists of two major genotypes, European genotype (Type 1) and North American genotype (Type 2), each formerly located on different continents. More recently, Type 1 PRRSV isolates (North American Type 1) have been identified in U.S. swineherds. This group of viruses possesses unique antigenic and genetic characteristics that are distinct from typical North American and European type PRRSV. A unique 51 by deletion has been identified in the immunodominant region of the Nsp2. The etiologic agent of PRRS is a small, enveloped virus containing a single positive-stranded RNA genome. PRRSV belongs to the family Arteriviridae, which includes equine arteritis virus (EAV), lactate dehydrogenase-elevating virus (LDV), and simian hemorrhagic fever virus (SHFV) (Snijder & Meulenberg, 1998). Nucleotide sequence comparisons showed that PRRSV can be divided into distinct European (Type 1) and North American (Type 2) genotypes (Allende et al., 1999; Nelson et al., 1999).
The PRRSV genome is about 15 kb in length and contains nine open reading frames. The 3′ end of the genome encodes four membrane-associated glycoproteins (GP2a, GP3, GP4 and GP5; encoded by sg mRNAs 2-5), two unglycosylated membrane proteins (E and M; encoded by sg mRNAs 2 and 6) and a nucleocapsid protein (N; encoded by sg mRNA 7) (Bautista et al., 1996; Mardassi et al., 1996; Meng et al., 1996; Meulenberg & den Besten, 1996; Meulenberg et al., 1995; Mounir et al., 1995; Wu et al., 2001, 2005). The replicase-associated genes, ORF1a and ORF1b, situated at the 5′ end of the genome, represent nearly 75% of the viral genome. The ORF1ab encoded polyprotein pp1ab is predicted to be cleaved at 12 sites to form 13 products: nsp1α, nsp1β, and nsp2 to nsp12 (Allende et al., 1999; den Boon et al., 1995; Nelsen et al., 1999; Snijder & Meulenberg, 1998).
Modified-live attenuated vaccines against PRRSV are currently available for reduction of clinical disease associated with PRRSV (Boehringer-Ingelheim Animal Health, Inc.). However, they cannot be distinguished serologically between pigs that have recovered from a natural infection and those that have been vaccinated. A genetically marked vaccine would allow the differentiation between vaccinated and naturally infected pigs, which is needed for PRRSV control and eradication programs.